As some have already mentioned here, it's Japanese Domestic Market as far as parts. However, this has now been also "made" into an "auto style" if you'd want to call in that. In other words, "JDM-styled" cars can be just about any Japanese-made vehicle (even mini-vans and micro-compacts) that's been modified to reflect the different parts that can be added to reflect the car culture or following for that particular brand in Japan. In a way, some of these cars reflect a more simpler approach to design and purpose than the "ricer" stage that have permeated the car scene since the "Fast & Furious" movies came into being.